It’s a false assumption that retinafying an image by doubling its resolution results in a file four times larger. Image compression doesn’t work like that – it hardly scales linearly with the amount of pixels in the image.
In PNG images there are huge areas of transparency, of single colors, and of repetitive color patterns that do not change. Because of that, actual file sizes of images that are twice the resolution of the original do not mean twice the size.
For JPEG images, just double the resolution and lower the JPEG quality. On a normal density screen this would cause ugly artifacts, but on a retina screen the pixels are so small you can’t see them.
Here is a WordPress plugin the creates the image files required by Retina (high-DPI) display: